This invention relates to circuits for adjusting the frequency response and other parameters of a high fidelity audio system and, particular, to a circuit for performing such adjustment automatically without special test conditions.
The quest for better fidelity in audio systems began with Thomas A. Edison and will probably continue forever, partly because the word "fidelity" is somewhat subjective. As used herein, fidelity relates to how accurately the sound adjacent a listener's ear corresponds to an electrical signal derived from a source of program material such as a microphone, a phonograph record, a compact disk, or a magnetic tape.
It has long been recognized in the art that distortions can arise not only in the electrical signal but in the loudspeakers and in a room itself. Typically, the prior art provides a compensating system including an equalizer (or a "graphic equalizer"), a microphone, and a spectrum analyzer. A test signal, such as "pink" noise or pulses, is converted into sound by the loudspeakers and the microphone converts the sound to an electrical signal for analysis. The equalizer is adjusted to minimize the unevenness in frequency response caused by the loudspeakers and by the acoustics of the room in which the test takes place. U.S. Pat. No. 3,732,370 (Sacks) discloses such a system.
U.S. Pat. No. 5,386,478 (Plunkett) describes a system in which a microphone senses a test signal from individually driven speakers and provides control information to a command module for adjusting an equalizer in a stereo.
While not discussed in the prior art, such compensating circuits are somewhat fastidious. For example, the room must be silent during a test. Any noise, i.e. any sound other than the test signal, interferes with and obviates the test. The room should be set up as it will be during use, including the location of furniture and the number of people. The test signal must be listened to in silence by the occupants of the room during a test. One can imagine listening to pink noise, which sounds like inter-channel hiss in an FM radio, or to pulses (popping noises), as the system is tested, speaker by speaker, frequency band by frequency band. Any substantial change in the listening environment, such as opening or closing draperies, requires that the test be performed again, which may not be convenient.
In a compensating system such as described in the Sacks patent, the system attempts to flatten the frequency response of the room acoustics, including the loudspeakers, by increasing or decreasing the amplitude in certain band. A listener who prefers or needs mid-range frequencies boosted is unable to make the necessary corrections without nullifying the settings determined by the test or, perhaps, making the system sound worse than before the test.
Compensating systems of the prior art are expensive. While such systems could be used to improve the fidelity of inexpensive stereo systems, one would be in the anomalous position of spending several times the cost of the stereo on a circuit to improve the sound of the stereo. As used herein, "stereo" is generic for a high fidelity audio system, regardless of the actual number of channels or speakers.
The prior art typically describes a compensating system that is in addition to an existing stereo. As audio systems becomes more compact, such additional equipment becomes aesthetically displeasing.
It is known in the art to modulate an audible sound with an inaudible sound for detecting feedback in audio systems. As disclosed in U.S. Pat. No. 5,649,019 (Thomasson), the inaudible sound is a replica of the original sound. If feedback occurs, the replica is recovered and is used to reduce the amplitude of the echo.
In view of the foregoing, it is therefore an object of the invention to provide an apparatus and a method for automatically adjusting a high fidelity sound system for room acoustics without a test signal.
A further object of the invention is to provide a compensating circuit that allows tone preferences.
Another object of the invention is to provide a compensating circuit that produces a sound at a location in a room that accurately represents an electrical signal derived from a source, even if the electrical signal includes modifications by a tone control circuit such as a single band filter or an equalizer having several bands.
A further object of the invention is to provide a compensating circuit that is transparent to a user during operation.
Another object of the invention is to provide a compensating circuit that can be incorporated into relatively inexpensive stereo systems.
A further object of the invention is to provide a technique for automatically adjusting a high fidelity sound system that is easily implemented in semiconductor devices incorporated into the sound system.